This invention relates generally to microwave circuits and more particularly to packaging of negative resistance diodes.
As is known in the art, a negative resistance diode, such as an IMPATT diode, is often employed in an oscillator or an amplifier to convert DC power to radio frequency power. IMPATT diodes are often employed in radio frequency applications where very high output radio frequency power at a very high frequency and relatively high conversion efficiency is required. As is well-known in the art, a plurality of radio frequency signals provided from a like plurality of IMPATT diode sources arranged in an appropriate manner may be combined into a composite signal by being coupled to a common resonant cavity. A portion of the composite signal is then coupled from the cavity to provide the output signal. In a conventional power combiner, each diode is mounted in a coaxial line amplifier or oscillator structure and a plurality of these coaxial structures are then coupled to sidewall portions of the resonant cavity. These coaxial structures are generally one of the more difficult elements to fabricate in the power combiner.
The coaxial line element generally includes an outer conductor dielectrically spaced from a centrally disposed inner conductor which provides in combination a coaxial transmission 1ine. Typically, a tapered sleeve comprising a lossy plastic material is disposed around one end portion of the center conductor to provide a matched termination load for the transmission line. The opposing end portion of the center conductor is attached to a first electrode of a packaged IMPATT diode. A second electrode of the packaged IMPATT diode is attached to the outer conductor by threading the packaged diode into a diode holder which is a threaded portion of the outer conductor of the coaxial line section. Prior to the packaged diode making contact with the center conductor of the coaxial transmission line, an annular member and spacer member are generally provided around the center conductor to provide a requisite impedance match between the IMPATT diode and the cavity and also to provide a predetermined distance between the diode and the midplane of the resonant cavity.
To obtain optimum performance from an IMPATT diode, the heat generated by the IMPATT diode must be efficiently removed from the IMPATT diode. Therefore, one package commonly used with such IMPATT diodes includes a thermally and electrically conductive pedestal support comprising a threaded stud portion at one end and an upper mounting plate at the other end. Often, a gold plated slab of diamond, which serves as a heat sink, is impressed into or bonded to the upper mounting plate. A ceramic hollow cylindrical shaped barrel is then bonded to the gold plated diamond and a beam-leaded diode chip is then disposed inside the barrel with the bottom of the diode being bonded to the gold plated diamond heat sink. The heat sink thus also forms one of the aforementioned electrodes of the device, and beam leads of the diode being bonded to upper portions of the gold plated ceramic barrel, forms the second one of the aforementioned electrodes of the device.
Several problems are generally associated with the above-described arrangement. The first problem involves providing the requisite impedance matching between the IMPATT diode chip and the resonant cavity. As is known in the art, the IMPATT diode chip is a relatively low impedance device. Therefore, matching the impedance of the IMPATT diode to the impedance of the cavity requires the use of the above-mentioned annular member which is disposed around the center conductor of the coaxial transmission line and acts as an impedance transformer. In using the annular member, the diode chip must be coaxially aligned within the center of such member. This in turn requires that the ceramic barrel be mounted in the package in coaxial alignment with the threaded stud and the cylindrical plate. This is generally a difficult task to perform, however, requiring precise machining of the respective components. Additionally, there is an electrical problem associated with this type of design since there must be sufficient electrical contact around the circumference of the annular member in the region where such member meets the shoulder of the diode package. This region is a very low impedance region, and small gaps in this region may cause r.f. leakage and changes in diode impedance which may significantly degrade device performance. Therefore, for good contact, both the contacting surface of the annular or transformer member and the surface of the package shoulder must be flat and smooth. Since the surface of the package shoulder is formed by the facial surface of the upper plate and the diamond heat sink, the thickness of the diamond heat sink must be substantially equal to the depth of the machined hole provided in the upper plate, otherwise, the diamond slab will be slightly recessed or will slightly protrude. Further, the force imparted to the diamond to press it into the machined hole must be uniform; otherwise, the diamond slab will be tilted. If the diamond is recessed or protrudes or if tilting occurs, it is generally difficult to provide good electrical contact between the diamond heat sink and the transformer members. Further, the edges of a protruding diamond heat sink may damage the corner of the transformer as the package is torqued into place and good mechanical and electrical contact between the members may not be provided.
A second problem associated with this type of design involves the thermal transfer efficiency required to remove heat generated by the diode. Since the package requires extensive machining during its manufacture, it must be made from copper alloys which are relatively hard, and therefore, have lower thermal conductivity than the diode holder portion of the outer conductor. Further, since the package is screwed into place to make electrical contact with the center conductor of the coaxial line, a gap is often introduced between the underside of the package shoulder and the diode holder. Heat must travel down the threaded stud portion and then across the threaded interface before being dissipated in the diode holder. Since the threads are under compression, there is thermal contact only over about one-half of the available area, thus reducing efficiency of thermal transfer.
Handling precautions are an additional problem associated with this type of design because of the relatively small dimensions and special materials used in the parts of the diode package. For example, the ceramic barrel is not protected mechanically after the package has been assembled and therefore may be easily disturbed or damaged. Further, during assembly of the coaxial element, the center conductor is insulated from the outer conductor by an annular member machined from a material generally known as Rexolite. This material has a low r.f. dielectric loss. However, this material is very brittle when machined into small parts, and thus the breakage rate of these parts is often high during assembly of the coaxial elements. Finally, the threads on the stud portion must generally be formed in a copper alloy material. This material does not provide machined threads which can generally withstand the high torque encountered in mating the diode package to the transformer member during manufacture of the coaxial section without distorting the threads. Often, electrically good diodes are lost when stripped threads on the package make the diode unusable.